1. Field of the Invention
The present invention relates to a lithium ion secondary battery.
2. Description of the Related Art
In recent years, lithium ion secondary batteries have been noted particularly as secondary batteries having high energy density, and their research, development, and commercialization have been progressed rapidly. At present, lithium ion secondary batteries have been generally popularized as small-sized domestic use for mobile phones and notebook personal computers. Further, large-scale secondary batteries of higher capacity than usual have been demanded as batteries for domestic, industrial, on-board uses etc. in view of problems or issues such as global warming, fuel exhaustion, and denuclearization.
As measures of increasing the capacity of the lithium ion secondary batteries, it has been known to use silicon (Si) or Si-containing compounds for negative electrode materials. However, Si involves a problem in view of rate characteristics since Si has a powder resistance which is two orders of magnitude higher than that of graphite. Further, increase of the coating amount of the electrode has not been investigated since Si involves a problem in the cycle life.
JP-2008-277232-A describes use of a negative electrode material for a lithium secondary battery, the negative electrode material disposing carbonaceous material particles containing at least one element selected from graphite and carbon black on surfaces of composite particles containing graphite particles, fine Si particles and amorphous carbon (A), in which the carbonaceous material particles are coated with amorphous carbon (B), in order to increase initial charge/discharge capacity and improve charge/discharge cycle characteristics of the lithium secondary battery. The configuration described above can moderate a volume change of the fine Si particles and maintain an electric contact of the active material during charge/discharge, thereby improving charge/discharge cycle characteristics.